The term xe2x80x9cwet compressionxe2x80x9d is used to describe compression of a gas wherein liquid in contact with the gas being compressed is evaporated by the heat of compression.
Evaporation of the liquid results in a lower temperature rise for the gas during compression than would occur without evaporation. This effect is also known as xe2x80x9cadiabatic coolingxe2x80x9d, that is, reduction of temperature without removal of heat.
The lower temperature of the gas at the end of compression results in lower energy required for the compression. This provides an economic benefit in the form of lower power cost.
The term xe2x80x9cisothermal compressionxe2x80x9d, or compression at constant temperature, is a more general term for the idealized embodiment of this process and is used in the title of reference U.S. Pat. No. 4,478,553, above.
Wet compression can be applied to the compression of most gases in most kinds of compression equipment. However the greatest potential economic benefit and greatest savings in energy cost will be in the application of wet compression in gas turbines.
It is well established that the efficiency of a gas turbine can be improved by the use of wet compression as described in the Kleinschmidt reference, above.
Among the conclusions by Shao-Lee Soo in a publication in Other References, above, is the statement xe2x80x9cEfficient wet compression requires water particles of size below 20 microns diameter. There is little or no danger of erosion of blades with particles of this order of magnitude.xe2x80x9d Droplets of this small size in a gas are often referred to as a mist or fog.
A mist or fog can be created by forcing liquid through a nozzle specifically designed for this purpose. Bete Fog Nozzle Inc. and Spraying Systems Co. are companies that manufacture such nozzles.
A problem for this requirement is that all nozzles produce a spray of droplets of varying sizes. If a nozzle is designed and operated so as to produce droplets of 20 microns in diameter some droplets will be 20 microns or less but there will be larger droplets as well. The larger droplets can cause erosion in the compressor and will not evaporate completely thus limiting the benefits of wet compression.
In the invention of U.S. Pat. No. 5,930,990 the nozzles specified as preferred are 1-7N-316SS12 from Spraying Systems Co. These nozzles do in fact produce a spray with droplets of varying size. According to data received from the manufacturer of these nozzles, Spraying Systems Co., the drop size under the conditions specified will vary from 7 microns to 501 microns. About 98% of the volume of droplets will be larger than 20 microns, and about half of the volume of droplets will be larger than 165 microns.
Thus in this preferred embodiment of U.S. Pat. No. 5,930,990 it is to be expected that wet compression will not be efficient, that is, larger water droplets will pass through without being evaporated and there will be the danger of erosion by the larger droplets.
Another problem in the application of wet compression is control of amount of water injected into the inlet air stream of the compressor. When starting wet compression water should be added in small increments of the total amount desired to avoid thermal stresses as pointed out in U.S. Pat. No. 5,930,990, a reference to this application.
If a fogging nozzle is operated at a liquid flow less than the design flow there will be a larger proportion of larger droplets and as the flow decreases there will be more larger droplets finally resulting in a discharge from the nozzle that is a solid stream of liquid with no droplets.
In the invention of U.S. Pat. No. 5,930,990 there is disclosed a spray rack group assembly in which there is a plurality of nozzles discharging into the inlet gas of the compressor. The patent discloses in claim 1 that liquid is added xe2x80x9cin a plurality of nebulized water mass flow increments . . . xe2x80x9d. The means of obtaining these increments is not shown or described but evidently water would be admitted independently to each spray rack pipe 303 in FIG. 4 of U.S. Pat. No. 5,930,990 by means of valves or by having a separate pump for each spray rack pipe. This would require a complicated system of valves and controls. In FIG. 4 there are shown five spray rack pipes meaning that there would be five increments available. Increments would have to be added in steps, not smoothly and continuously.
In the invention of reference U.S. Pat. No. 4,478,553 wet compression is conducted by introducing liquid, referred to as coolant, directly in to the gas flow through passages in the rotating elements of the compressor. The preferred embodiment of this invention uses a centrifugal compressor whereas most gas turbines use axial flow compressors. The invention can be applied to axial flow compressors but at greater complexity and expense. The specification does not disclose how the invention might be applied to an axial flow machine. Application of this invention to an existing machine would require a significant rebuilding of the machine.
It is therefore the object of this invention to provide means of creating droplets of liquid, carried in a moving stream of gas, in which the size of the droplets is controlled so as to be smaller than a given, predetermined size so that said stream of gas containing the liquid droplets can be utilized as the inlet flow of a gas compressor in which the droplets will not cause erosion and will evaporate completely in the gas compressor.
It is a further object of this invention to provide a means of controlling the flow of liquid droplets of a predetermined size smoothly and continuously from zero flow to the maximum flow desired for the application.
It is a further object of this invention to provide means of creating droplets of liquid, carried in a moving stream of gas, in which the size of the droplets is controlled so as to be smaller than a given, predetermined size so that said stream of gas containing the liquid droplets can be utilized as the inlet flow of any device in which said stream of gas an liquid can be usefully employed.
In the application of this invention this desired result is accomplished by first dispersing a liquid in a stream of gas by using known means such as fogging nozzles and then removing from the stream of gas droplets that are larger than desired for the particular application.